The invention relates to a plasma spraying method. The generic process, described in U.S. Pat. No. 5,853,815, is a thermal spraying for the manufacture of a so-called LPPS (=low pressure plasma spraying) thin film. The invention also relates to applications of the method and to components which are coated in accordance with the method in accordance with the invention.
A conventional LPPS plasma spraying process is modified in the technical process by the LPPS thin film process, with a space through which plasma flows (“plasma flame” or “plasma beam”) being expanded due to the changes and extended to a length of up to 2.5 m. The geometrical extension of the plasma results in a uniform expansion—a “defocusing”—of a powder beam which is injected into the plasma with a delivery gas. The material of the powder beam, which disperses to form a cloud in the plasma and is partly or fully melted there, reaches a widely extended surface of a substrate in a uniform distribution. A thin film is formed on the substrate having a coating thickness that is less than 10 μm which forms a compact covering due to the uniform distribution. A thicker coating with special features can be produced by a multiple application of thin films, which makes such a coating usable as a functional coating. A porous coating can, for example, be produced using a multiple application and is suitable as a support for catalytically active materials (see EP-A-1 034 843).
A functional coating, which is applied to a base body forming the substrate, as a rule includes different part coatings. For example, for a gas turbine (stationary gas turbine or airplane power plant), which is operated at high process temperatures, the blades are coated with a first single coating or multi-layer part coating which produces resistance to hot gas corrosion. A second coating, which is applied to the first part coating and for which ceramic material is used, forms a heat insulating coating. The LPPS plasma spraying process is suitable for the manufacturing of the first coating. The heat insulation coating is advantageously produced with a process in which a coating with a columnar micro-structure is created. The coating thus structured is composed approximately of cylindrical small bodies or particles whose central axes are aligned perpendicular to the substrate surface. Transitional zones in which the density of the deposited material is lower than in the particles bound the particles at the side. A coating which has an anisotropic micro-structure in this manner is tolerant in expansion with respect to changing strains which result due to repeatedly occurring temperature changes. The coating reacts to the changing strains in a largely reversible manner, i.e. without forming cracks, so that its service life can be considerably extended in comparison with the service life of a customary coating having no columnar micro-structure.
The anisotropic micro-structure can be produced using a thin-film method such as a vapor deposition method. In this process, which is termed “EB PVD” (electron beam physical vapor deposition), the substance to be deposited for the heat insulating coating is brought into the vapor phase by an electron beam in a high vacuum and condensed from the phase onto the component to be coated. If the process parameters are suitably selected, a columnar micro-structure thus results. A disadvantage of this vapor deposition process is the high plant cost. In addition, the same tool cannot be used for the LPPS plasma spraying process and for the EB PVD process in the manufacture of a coating including a plurality of partial coatings. A plurality of work cycles must therefore be carried out for the coating.